Machinery's Handbook, 31st Edition
Production of Metallic Powder 1531 Centrifugal Atomization: Centrifugal atomization of molten metals is a cost-effective process for powder production and spray deposition. The properties of the powder and deposit produced by this method are determined primarily by the characteristics of the atomized droplets, which in turn are largely dependent on the flow development of the melt on the atomizer. There are basically two types of centrifugal atomization processes: the rotating disk process and the rotating electrode process. Rotating Disk Process: In this process, the molten metal stream drops onto a rapidly rotating disk or cup on a vertical axis so that centrifugal forces break up the molten metal stream to form, first, a film of molten metal and then to generate particles ( Fig. 7a). Naturally, the material of which the spinning disk or cup is made takes a lot of punish ment. Particle sizes are not very fine, unless very high speeds (over 10,000 rpm) are used. An advantage of this process is that it can produce much narrower distributions than gas or water atomization. Thus, excessively fine or oversized particles can be minimized. Molten Metal
Inert Gas
Vacuum
Drive Belt
Spindle
Rotating Disk
Motor
Rotating Electrode
Chamber
Nonrotating Tungsten Electrode
Metal Powders
Drive Shaft
Powders Collection
Fig. 7. Centrifugal Atomization: (left) Rotating Disk Method; (right) Rotating Electrode Method Rotating Electrode Process: In the rotating electrode process (REP), an arc is struck between a horizontal tungsten cathode and a spinning electrode of the desired metal for pulverization (Fig. 7b). The electric arc (or plasma in later versions) melts the spin- ning workplace electrode tip, which throws off a shower of the molten metal droplets by centrifugal forces. The droplets solidify as they pass through a cooling gas or vacuum and are collected in a concentric chamber. A salient characteristic of REP powder particles is their perfectly spherical shape and freedom from satellite particles adhering to them. Mechanical Processes.— There are basically two types of mechanical processes for the production of metal powders: the mechanical comminution process and the mechanical alloying process. Mechanical Comminution Process: Brittle materials such as ferro-alloys (ferro-chro- mium, ferro-silicon, etc.) are pulverized mechanically in ball mills (Fig. 8). A process known as the coldstream process is finding increasing application for the production of very fine powders such as are required for injection molding. Milling is the primary method for reducing the size of large particles and particle ag- glomerates. Ball, hammer, vibratory, attrition, and tumbler mills are some of the com mercially available comminuting devices. During milling, forces act on the feed metal to modify the resultant particles. Impact, attrition, shear, and compression all influence powder particle size and shape. Lathe turning is a technique used for materials such as magnesium for creating coarse particles from billets. These particles then are reduced in size by milling or grinding.
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